David Cameron has welcomed a widow to Downing Street on live television and hailed her as the "best of British" for fostering 300 vulnerable children.

Doreen Roberts was taken for a surprise meeting at the Prime Minister's home in a silver Jaguar, with Mr Cameron calling for society to recognise what she had done for children.

Over the last 47 years Mrs Roberts, 78, from Langport in Somerset, and her late husband Don fostered around 300 children, as well as bringing up five children of their own.

The Prime Minister, who launched the Give a Child a Home campaign last October to encourage adoption and fostering, said she was an example of the "big society".

Opening the door of 10 Downing Street to her, he said: "Come in, it's freezing. I just want to say thank you for what you have done, it's pretty amazing."

It was part of the ITV programme Text Santa, a Christmas show which is helping UK charities raise money for their causes.

Mr Cameron said: "Doreen and her late husband Don are great examples of the best of British.

"I hope her story will inspire others. It brings home how much joy children bring to our lives and shows how we can support them.

"As a government we are committed to do more to help people adopt and foster. It is shocking that of the 3,600 children under the age of one in care, only 60 were adopted last year and there are now 65,000 children in the care system.

"We need more people to think about providing a child with a loving home, with the tenderness and stability that a family brings."